It is known that useful polymers which are either elastomeric or non-elastomeric in general properties may be produced in solution using such catalysts as lithium hydrocarbyls, specifically lithium alkyls. Once having synthesized the polymer in solution, it then becomes necessary to provide an adequate means and apparatus for isolating the polymer from its solvent. With certain polymers such as conjugated-diene-homopolymers, this may be effected by contacting the polymer solutions (cements) with saturated steam under relatively low total mass flux conditions designed to flash off the bulk of the solvent, dropping the polymer crumb so formed into a hot water bath to degas the polymer, draining the water and subjecting the polymer crumb to drying conditions such as in a moving belt tunnel arrangement. While this is satisfactory for certain polymers, the crumb so formed is relatively coarse and tends to retain substantial proportions of the solvent as well as to be highly saturated with water at the beginning of the drying operation. Moreover, many polymers cannot be treated this way since they tend to agglomerate due to the combination of residual solvent and elevated temperatures so that the products obtained are large sticky masses which are essentially impossible to effectively devolatilize and dry.
To overcome this problem a low cost process for separating and recovering polymer particles was invented which comprised: (a) feeding steam into a high shear mixing zone of the central section of a cylindrical casing, (b) feeding a film of polymer solution having a viscosity of 20-10,000 cp. into the high shear mixing zone, the solvent of said solution having a maximum boiling point below the temperature of the steam, the steam temperature being below that at which the polymer will show evidence of appreciable decomposition under the conditions of high shear contact, the ratio of steam to solution and the residence time in the mixing zone being sufficient to vaporize at least about 90% of the solvent, whereby the polymer is isolated from solution as powdered particles; (c) contacting the steam and solution under high shear conditions; (d) passing the sheared mixture to a cyclone separation zone wherein the powdered polymer which may either agglomerate or remain finely divided is separated from steam and vaporized solvent; (e) removing residual solvent; and (f) removing residual water from the particles. This process is described in U.S. Pat. No. 3,804,145, issued Apr. 16, 1974, which is incorporated by reference herein.
It is desired that the level of residual solvent that remains with the polymer be below about 2.0 phr (parts per hundred), preferably below about 1.8 phr and most preferably below about 1.5 phr in order to effectively operate the hot air dryers. If the residual solvent is above these levels there is a tendency to agglomerate and foul up the hot air dryers. To achieve this level in the above process it had previously been thought necessary to contact the cement and steam in the high shear mixing in a cement to steam weight ratio of about 1:1.5, preferably about 1:1.3 most preferably about 1:1. The maximum amount of steam necessary is determined by economical factors in part the less steam used the lower was the cost. But the minimum amount of steam used is dependent on the minimum amount of residual solvent that can be tolerated in the hot air drying stage of the process. Therefore in order to obtain a less than about 1.5 phr solvent level it has been found necessary to use between about 1.0 to about 1.5 pound of steam for every pound of cement.
Previously, all of this steam was passed into the cyclone separator via a high shear mixing chamber, i.e., the steam and polymer solution (cement) were contacted before entering the cyclone. The steam to cement ratio passing into the high shear mixer determined the size of the resulting polymer particle. Since the residual solvent level must be below about 2.0 phr in order to operate the hot air dryer, the steam to cement ratio passing through the high shear mixing chamber was thought to be fixed at about between 1:1 and about 1.5:1 thereby fixing the size range of the recovered polymer particle. In some cases the resulting particle was too small, resulting in difficulty in handling and transporting the particles. This problem was especially true in the case of the two-block styrene/hydrogenated-isoprene A-B block copolymers.
It has now been found that both the recovered polymer particle size and the residual monomer level are simultaneously controlled by introducing the amount of steam necessary to produce the desired solvent level simultaneously through two separated inlets. The steam to cement level entering the cyclone separator via the contactor is reduced to a point where the desired particle size is obtained. That part of the steam which was not part of the stream entering via the steam contactor is introduced into the cyclone by a separate steam inlet. This allows one to control the size of the particles, yet still maintain the through-put of the devolitization system and the minimum residual solvent level needed to efficiently operate the hot air dryer stage of the process.